The metal fine particle having a particle diameter of around several nm to several tens nm exhibits various physical and chemical properties different from those of a bulk metal. For example, as an optical property, it is known from old times that by a color developing mechanism called a plasmon absorption, the metal fine particle exhibits a particular hue corresponding to the type and the size of a metal, and a solution of the metal fine particle is used as a colorant of a coating or the like. Besides this application, the metal fine particle has widespread applications such as conductive pastes, transparent conductive films, high-density recording materials, light shading filters, chemical sensors, catalysts, light guiding members, gas barrier materials, light scattering-reflecting members, light diffusing members, photosensitive materials, photoelectric conversion elements, inks for inkjet, high dielectric constant materials, and fluorescent materials.
Although as the production method of such a metal fine particle, there can be mentioned a gas phase method and a liquid phase method, the liquid phase method is of lower cost and can easily obtain advantageous fine particles having a narrow particle size distribution. Generally, the liquid phase method is a method of reducing a metal ion with a reductant in a state in which an organic dispersant having affinity with a metal is added to a metal salt solution to prepare metal fine particles. Representative examples of the dispersant include citric acid, surfactants, low molecular weight compounds having a thiol group or an amino group, and polymers such as polyvinyl pyrrolidone.
Patent Document 1 and Non-patent Document 1 describe a preparation method of a metal fine particle using a thiol compound. The surface of the thus obtained metal fine particle is solidly coated with the thiol compound, and therefore, the metal fine particle can be recovered as a powder and can be re-dispersed in a solvent. Non-patent Document 2 describes a preparation method of a metal fine particle coated with a low molecular weight compound having a dithiocarbamate group. Thus, a compound having a functional group containing a sulfur atom has high affinity with a metal surface, so that the compound exhibits excellent characteristics as a dispersant of a metal fine particle. However, there is no example of evaluating dispersibility of such a compound in a polymer. Moreover, the compound having a functional group containing a sulfur atom has a problem of activity as a catalyst because of the sulfur atom contained.
A composition containing metal fine particles having a nanometer size is widely used, for example, in a catalytic synthesis of various chemical products and is further expected to be utilized in an environmental catalyst, a hydrogen storage and retrieval reaction, and the like. Although the chemical and physical characteristics of the metal fine particle depend largely on the shape and the size of the particle, generally, a metal fine particle having a nanometer size has high chemical reactivity and the metal fine particles are easily aggregated to become a larger size to lose the characteristics thereof (Non-patent Document 3).
For preventing the above phenomenon, there is a method of dispersing metal fine particles on a solid carrier. For example, as the carrier, there is used a carrier having a pore structure and having a large surface area such as silicon dioxide, aluminum oxide, and zeolite. However, the metal fine particle is supported on such an inorganic solid, and therefore, the metal fine particle cannot be dispersed in an organic solvent, an aqueous solvent, or a resin.
Patent Document 2 describes a dispersion containing metal nano-particles in a dispersion formed by a polymer compound having a branched polyalkyleneimine chain in a solvent. Patent Document 3 describes a colloidal solution using a quaternary ammonium salt as a stabilizing agent for protecting colloidal particles.
Patent Document 4 and Patent Document 5 describe a nanometer-sized metal catalyst dispersed in a dendrimer or a hyperbranched polymer, and Patent Document 6 describes a hyperbranched polymer having an ammonium group at the terminal position of molecule. However, in these Documents, the stability thereof as a powder and the dispersing properties thereof in an organic solvent/aqueous solvent are not evaluated nor referred to, and there is no example of reporting a complex of a metal fine particle stabilized by a branched polymer material having an ammonium group.